"I'll stop making all of our conversations about Evan, I promise, but I thought this was just wonderful and told him as much yesterday," Costolo said in a message to Lynton, linking to a speech Spiegel delivered.

Costolo continued, "I really think he is one of the best product thinkers out there right now.

"Thoughtful, understands what the shifts in the landscape actually mean for people, clearheaded about what the implications are for product and design. Great."